Hello,
We have created a single DLL file that gives our application interface to telephony functions.
Attached only part of the DLL so it will not compile but should give you an idea of the task.
You will need to add to this DLL option to dial using simple Voice modems. the modem should use sound card to transmit/receive voice conversation. You should also check that the DLL works properly with multiple modems that receives phone calls. Make sure Caller ID and conversation recording works properly.

You can use any TAPI component you like as long as it works properly.

The DLL must detect Ringing/Answer event, we will provide detection code.

A developer from our side will be available for any help you will need.